1. Field of the Invention
The present invention relates to a magnetic recording medium and, more particularly, to a magnetic recording medium having a coating layer in which a binder having improved properties is used.
2. Brief Description of the Prior Art
So far, the vinyl chloride-vinyl acetate copolymers, cellulose derivatives or polyester resins have been used extensively as the binder for the magnetic recording medium. The thermoplastic polyurethane resin has also been used for improving wear resistance or controlling coating properties of the magnetic layer of the recording medium.
On the other hand, the magnetic recording medium is required to satisfy the demand for high density recording. As a result thereof, attempts have been made towards reducing the size of the ferromagnetic powders to be filled in the magnetic layer and towards improved smoothness of the surface of the magnetic layer.
However, with improved surface smoothness of the magnetic layer, its contact area is increased, so that its running performance and durability are affected, while its anti-blocking performance is also drastically lowered. Above all, since the conventional binder has a low softening point and inferior heat resistancy, when the magnetic recording medium in the tape form is taken up and stored on a reel under elevated temperatures or for a prolonged time, it is liable to become tightly affixed to the non-magnetic substrate neighboring to the magnetic layer. Thus, due to the resulting exfoliation of the magnetic layer, the properties of the magnetic recording medium are not exhibited satisfactorily. In addition, an increase in the specific surface resulting from comminution of ferromagnetic powders not only results in the drastically lowered dispersibility of the powders in the binder and deteriorates surface gloss and filling properties, but also renders it difficult to prevent powder debris or tape injury or to provide sufficient running durability or electromagnetic properties or characteristics that are critical to the magnetic recording medium.
Hence, in order to elevate heat resistancy of the thermoplastic polyurethane resin and thereby improve anti-blocking of the magnetic recording medium, it has been proposed to use the thermoplastic polyurethane resin as the binder for the magnetic layer of the recording medium, wherein the ratio of the low molecular weight diol in the thermoplastic polyurethane resin is increased for elevating the concentration of the urethane groups in the molecule.
The thermal properties of thermoplastic polyurethane resins may be improved in general by increasing the concentration of the urethane groups. That is, the higher the concentration of the urethane groups in the molecule, the higher the softening point of the thermoplastic polyurethane resin and the lower its glass transition temperature. However, with increase in urethane group concentration in the thermoplastic polyurethane resin, it becomes undesirably insoluble in general-purpose solvents such as ketones, alcohols, esters, aromatic hydrocarbons or aliphatic hydrocarbons, it being only soluble in solvents of higher toxicity such as dimethylformamide or tetrahydrofuran. Moreover, dimethylformamide or tetrahydrofuran used as solvent for a magnetic paint for formation of the magnetic layer may corrode surface portions of the substrate on which the paint is applied, or any surface portions or materials with which it may be brought into contact, so that creases or wrinkles are partially caused or, in some cases, these portions may become dissolved. Therefore, there is a limitation on improvement by elevating the concentration of urethane groups in the thermoplastic polyurethane resin.
Furthermore, even when the thermoplastic polyurethane resin with elevated concentration in the urethane groups is used as the binder for the magnetic layer, it has only negligible effects in improving the dispersibility of the comminuted ferromagnetic powders in the binder.